a) Field of the Invention
This invention relates to a microscope optical system adapted to make ordinary observations through a microscope and image recordings through photography or video.
b) Description of the Prior Art
Conventional microscope optical systems of the type, which have such a structure as shown in FIG. 1 by way of example, are constructed so that an imaging beam of light emanating from a specimen I.sub.1 and converged by an objective lens 1 is reflected in turn from a half mirror surface 2a and a first surface 2b (the surface normal to the optical axis of the objective lens 1) of an observing direction converting prism 2 to be bent obliquely upward (in an inclined direction) and causes an image I.sub.2 to be formed through a barrel prism 3 for observation of the image I.sub.2 through an eyepiece 4, while the imaging beam of light transmitted by a photographic prism 5 having a surface 5a parallel with the first surface 2b of the observing direction converting prism 2 and cemented to the half mirror surface 2a of the prism 2 causes an image I.sub.3 to be formed, which is reformed, as an image I.sub.4, on a film surface 7 through a photographic lens 6 to thereby photograph the image. Although the change of magnification of the photographic image I.sub.4 is generally carried out by converting the photographic lens 6 into that with different magnification, this arrangement renders it impossible to observe the photographic image whose magnification is varied and therefore has been improved in such a way that a beam splitter 8 is arranged between the photographic lens 6 and the film surface 7 and splits the imaging light beam traversing the photographic lens 6 to form a image I.sub.5 for being observed through a viewer eyepiece 9, thereby allowing the confirmation of the extent of the photographic image, namely, what is called framing, to be performed.
Such a structure, however, has the defect that a user is obliged to change his posture, whenever photography is carried out, in order to shift the position of his eye from the eyepiece 4 to the viewer eyepiece 9.
Hence, as an improvement of this defective structure, the arrangement shown in, for example, FIG. 2, is proposed. Specifically, this arrangement is such that variable magnification lenses 10 and 11 different in magnification from each other are removably disposed in the optical path of the objective lens 1 between the objective lens 1 and the observing direction converting prism 2 in order that the image I.sub.2 whose magnification is changed by the variable magnification lenses 10 and 11 is observed through the eyepiece 4, and such that a reflecting prism 12 is disposed to bend the imaging light beam transmitted through the photographic prism 5 for the formation of an image I.sub.6, which is reformed, as an image I.sub.7, on the film surface 7 by the photographic lens 6 and a reflecting prism 13, thereby allowing the image to be photographed. Here, the photographic lens 6 serves as a relay lens having no variable magnification function. Furthermore, the arrangement is made so that a reticle projecting system is disposed which comprises an illumination light source 14, a focusing plate 15 having a field frame and a reticle, and a reticle projecting lens 16 and whose optical axis coincides with the optical axis bent by the half mirror surface 2a of the observing direction converting prism 2 to form a reticle image at the position of the image I.sub.2, thereby enabling the image I.sub.2 whose magnification is changed and the reticle image to be observed within the same visual field through the eyepiece 4.
Although the image I.sub.3 is depicted in the figure, this merely shows that, if there is the need for automatic focusing detection by way of example, it is only necessary to form the image of an object in the optical path of the light beam transmitted through the reflecting surface of the prism 12, as a semi-transmissive reflecting surface, and therefore the image is not always formed at such a position.
With this arrangement, since the eyepiece 4 functions as a finder, the user does not need to change his posture whenever photography is performed.
The foregoing arrangement, however, has the disadvantage that since the variable magnification lens 10 or 11 is placed between the objective lens 1 and the observing direction converting prism 2, the position for observation becomes high with the result that observation is very difficult. For a zoom lens, the length in the direction of the optical axis is so great that its adoption is impossible in practical use. Further, although such a structure often involves the technique that a scale and a data indicating plate are arranged at the position of the image I.sub.6 and their images are photographed, together with the image I.sub.7, on the film surface 7, it has the defect that the positions of their photographed images cannot be viewed through the eyepiece 4. Moreover, in the structure, the field frame observed through the eyepiece 4 is bright against the background, namely, assumes a so-called bright frame. This, however, brings about the defect that where the image of the background is bright, the frame is extremely difficult of view.
Thus, as a technique to solve the preceding problem of the bright frame, the optical system, for example, set forth in U.S. Pat. No. 4,527,869, is proposed, which as shown in FIG. 3, comprises: a quadrilateral prism P having a first surface S.sub.1 and a second surface S.sub.2 normal to the optical axis of an objective lens L.sub.1 and a third surface S.sub.3 and a fourth surface S.sub.4 parallel with the surface on the eyepiece side of the observing direction converting prism, arranged to be replaceable with respect to the observing direction converting prism; a loop optical path comprised of first, second and third reflecting members M.sub.1, M.sub.2 and M.sub.3, arranged in the plane including the optical axis of the objective lens L.sub.1 so that a light beam transmitted by the first surface S.sub.1 and the second surface S.sub.2 of the quadrilateral prism P is reflected in turn from the reflecting members to be incident on the third surface S.sub.3 of the quadrilateral prism P, followed by successive reflections from the second surface S.sub.2 and the first surface S.sub.1, and coincides with the optical axis directed toward the eyepiece L.sub.10 ; a lens system L.sub.4 arranged in the loop optical path to reform an image formed by the objective lens; and a focusing plate P.sub.3 arranged at the position of the image I.sub.1 in the loop optical path, to take out the light beam for photography by arranging removably the second reflecting member M.sub.2 in a photographic optical path.
In this structure, however, the disadvantage is caused that, in ordinary image observation and photographic image confirmation, the subtraction between the numbers of times of reflections to which the light beam is subjected from the objective lens to the eyepiece gives an odd number, so that, in order to render the orientations of both images equal, the use of a Dach prism is required for the third reflecting member M.sub.3 with resultant complication of arrangement and high cost. Furthermore, if the arrangement is such that the photographic lens L.sub.7 is positioned between the first reflecting member M.sub.1 and the second reflecting member M.sub.2 and used as the zoom lens in order to enable the magnification to be changed without rendering the position for observation high and enable its situation to be viewed, the third reflecting member M.sub.3 will be positioned on a considerably lower side as it is farther away from the optical axis of the objective lens L.sub.1, with the result that bulky size of the lens barrel is inevitably brought about and the unity of the photographic device also becomes difficult.
Thus, as a measure of solving the preceding problem of the bulky size of the lens barrel, the optical system, for example, set forth in U.S. Pat. No. 4,547,047, is proposed, which is constructed to render an imaging light beam passing through a photographic lens incident on the side of a prism of a Jentzsch type lens barrel through a loop optical path comprised of a plurality of reflecting members arranged in a plane normal to the optical axis of an objective lens. With this structure, however, the loop optical system fails to be included in the same plane as the optical axis of the objective lens and it is therefore required that an arrangement is made so that an image with no inclination can be observed. Although, for this reason, an image rotating optical system is proposed, it also has the defect that the structure is complicated with the resultant high cost. Moreover, the structure has another defect that the optical system is not applicable to a Siedentopf type lens barrel in which the angle subtended by the optical axis on the side of the prism of the lens barrel varies in interpupil distance adjustment. Thus, any of the conventional microscope optical systems capable of photographing the image has both merits and demerits and fails to fulfil the requirements that the size is small to be easily used and the structure is simple.